Dramatic worldwide increases in antibiotic resistance raise the specter of a return to the pre- antibiotic era. Overuse of antibiotics is the main driver of this process. One of the main areas of clinical medicine where unnecessary antibiotic use occurs is in the management of patients with acute respiratory infections, which are often caused by viruses. The long-term goal of our work is to develop better diagnostic tests that will help physicians avoid using antibiotics to treat viral infections. Highly sensitive molecular assays are now available to detect respiratory viruses. However, their high sensitivity leads to detection of viruses in cases in which they are not the cause of the patient?s illness. We have proposed using host response, including host gene expression, to provide more accurate diagnostic information. Most previous work evaluating host response to respiratory infections was done using blood samples but we have recently shown that human gene expression can be measured in nasal samples, and the ability to recognize and discriminate between symptomatic infection, asymptomatic infection, and no infection is at least as good for nasal samples as for blood. The work proposed here represents an academic-industry partnership between Washington University and BioFire Diagnostics that will develop a new diagnostic test performed on a nasal sample that combines virus detection with human gene expression. Our intention is to move the test towards regulatory approval so that it can be used to help patients and physicians. BioFire will prepare a prototype test device using their FilmArray system, which is already widely used for multiplex pathogen detection. The viral component is based on assays that BioFire has developed, and the gene expression component will use genes selected by the Washington University team based on our recent with informed consent/assent. A nasal swab from each subject will be tested using the prototype test device and a blood sample will be obtained for procalcitonin measurement to help determine the cause of the patient?s illness. The results of the prototype test device will be compared to classification by a panel of expert clinicians who will use the patient?s clinical information including the procalcitonin result to determine whether the patient had a viral infection, bacterial infection, both, or neither. We will also analyze the potential impact of using the test results on antibiotic utilization. In a second phase of the study, samples that are discrepant between virus detection and host response will be analyzed using RNA-seq and quantitative virology to understand why results were discrepant. If this project is successful, it will lead directly to a clinical trial designed to establish clinical efficacy, used to support an application for FDA approval.